Both are virtually identical, aside from the two differences you mentioned. Bugs and features are very likely to affect both exactly the same.
The NBG6817 might be slightly easier for beginners to flash (starting with master or >=19.03.0) from the OEM webinterface (while the r7800 requires tftp, due to the changed partitioning).
The r7800 provides you with 4+96.5 MB usable space on NAND, the NBG6817 with 2*(4+64) MB in a dual-boot setup on eMMC (which can be quite nice, as it allows reverting to the previous firmware installation with its configuration untouched).
Both have robust push-button tftp recovery mechanisms (the r7800 needs you to push the factory firmware with a tftp client to the router, the NBG6817 pulls it from a tftp-server you provide).
Unless you're looking for a particular feature specific to one of them, you can just base your decision based on availability and prices, both are a good choice for WAN speeds up to 350-400 MBit/s and quite performant for tasks beyond that.